Substantial efforts have been made in recent years toward improving the efficiency and yields of semiconductor solar cells and reducing their production costs. As compared to silicon, gallium arsenide has long been recognized as a material with a bandgap more favorable for maximizing solar cell efficiency and also a material better suited for operation at elevated temperatures. Additionally, because of the small optical absorption depth in direct transistion semiconductors like gallium arsenide, short minority carrier diffusion lengths can be tolerated, and this leads to better resistance to radiation damage.
One state-of-the-art type of gallium arsenide solar cell which has evolved as a result of continuing efforts in solar cell research is a gallium arsenide solar cell comprising an N-type galium arsenide substrate upon which a P-type layer of gallium aluminum arsenide is epitaxially deposited. One such structure is shown, for example, in an article by J. DuBow in Electronics, Vol. 49, No. 23, Nov. 11, 1976 at page 89. Another such structure is disclosed by H. J. Hovel et al in an article entitled "Ga.sub.1-x Al.sub.x As - PPN Heterojunction Solar Cells", Journal of the Electrochemical Society, September 1973 at pages 1246--1252, and both of these articles are incorporated herein by reference. Yet another such solar cell structure is disclosed in a copending application of Sanjiv Kameth et al., Ser. No. (PD-76165) assigned to the present assignee.
While the above type of gallium arsenide-gallium aluminum arsenide solar cell has proven to be a relatively efficient device and generally satisfactory in several respects, it nevertheless requires a relatively thick bulk gallium arsenide substrate starting material for its manufacture. This requirement is partially responsible for the expensive manufacturing cost for these solar cells as a result of the high cost of gallium. However, it is well known that major efforts are continually being made in both silicon and gallium arsenide solar cell research to reduce solar cell manufacturing costs by providing high yield and large volume manufacturing processess for economically making these solar cells.
In addition to the above efforts to reduce solar cell costs, there are also continuing efforts to increase the power conversion efficiency of these cells. In this regard, it has been observed that the gallium aluminum arsenide window material of the above state-of-the-art type gallium arsenide - gallium aluminum arsenide solar cell is somewhat limited in its transmission of the blue spectrum of sunlight. This, of course, is a limiting factor as regards the maximum attainable efficiency for these cells as well as the ultimate cost reduction of large scale solar cell arrays.